1. Field of the Invention
The present invention relates to a method of making an optical element, and more specifically to an injection-molding method for making an optical element with advanced precision. The optical element so formed can be suitable for use in an image forming apparatus such as a laser beam printer, a digital copying machine, or a multifunction printer having an electrophotographic process.
2. Description of the Related Art
An optical scanning apparatus of a laser beam printer (LBP) or a digital copying machine typically includes an optical deflector (deflecting unit) in the form of a rotatable polygon mirror which periodically deflects a light beam that is optically modulated and emitted from a light source unit according to an image signal. The deflected light beam is converged to form a spot on the surface of a photosensitive recording medium (photosensitive drum) by an imaging optical system having an fθ characteristic. More specifically, the deflected light beam passes through the imaging optical system and is focused to form a spot-like image on a surface of a photosensitive recording medium. In order to maintain optimal imaging precision, the axis of the focused beam must be perpendicular to the surface of the photosensitive recording medium. In other words, the imaging optical system provides a flat image field at the surface of interest. By optically scanning the surface of the photosensitive recording medium, image recording is performed. However, since the surface of the photosensitive recording medium is the cylindrical surface of the photosensitive drum, it is difficult to maintain a flat image field at the surface of interest.
In this type of optical scanning apparatus, a plastic lens, which is easy to make and lightweight, is often used in an imaging optical system. A plastic lens is characterized by being suitable for mass production by injection molding.
In order to improve precision on the curvature of field on the surface of a photosensitive drum, to reduce the scanning line curvature on the surface of a photosensitive drum, and to improve the fθ characteristic, the optical functional surface of a plastic lens is often designed to have an aspherical shape. For this reason, the optical functional surface of a plastic lens needs to be formed by injection molding in a desired aspherical shape that can meet the highest desired precision.
However, it is known that when a plastic lens is formed by injection molding, precision on the surface shape of the completed lens is deteriorated by shrinkage. For example, when a lens is formed of plastic, the completed lens is smaller than the size of a cavity formed by an optical insert. The shape of the optical functional surface is also deformed by molding shrinkage, relative to the surface shape of the optical insert. If such an error is not within a design allowable range, the molded element cannot be used as a product.
On the other hand, if the displacement from a mold occurring at the time of molding is stable and does not vary greatly depending on the molding date and time and environment, and if the error is preliminarily corrected by the mold shape, the shape of a molding can be kept within the design allowable range and the molding can be used as a product.
Various methods for making an optical insert that take into account shrinkage and deformation at the time of molding have been proposed.
Japanese Patent Laid-Open No. 7-60857 discloses a method including forming a lens, measuring the shape error of the optical functional surface of the lens, and modifying an optical insert of a mold so as to compensate for the shape error due to the influence of nonuniform shrinkage of resin. Japanese Patent Laid-Open No. 2002-248666 discloses a method including correcting the shapes of some of optical functional surfaces so as to compensate for the curvature of field on the basis of the measurement result of optical characteristics.
In recent years, an optical system (for example, an oblique incidence optical system) in which a light beam (light ray) does not pass through a meridional line including a lens optical axis in an imaging optical system has been widely used for the purpose of making the whole apparatus compact. It is known that in such an optical system, the wavefront aberration in the direction of 45 degrees (hereinafter referred to as 45-degree astigmatism) varies depending on the shape of the sub-scanning cross section (the normal angle in the sub-scanning direction at a position where a light ray passes through the optical functional surface of a lens).
Japanese Patent Laid-Open No. 2002-248666 discloses redesigning the partial curvature in a particular optical functional surface so as to correct the curvature of field obtained in the optical focus measurement. However, it does not disclose the inclination at the position through which a light ray passes. For this reason, in the method disclosed in Japanese Patent Laid-Open No. 2002-248666, the 45-degree astigmatism may deteriorate, and the spot on the surface of the photosensitive drum may enlarge.
In addition, the height reached by a light ray on the surface of the photosensitive drum varies depending on the shape of the sub-scanning cross section (the normal angle in the sub-scanning direction at a position where a light ray passes through the optical functional surface of a lens). For this reason, in the method disclosed in Japanese Patent Laid-Open No. 2002-248666, the scanning line curvature may be deteriorated by change of the height reached by a light ray on the surface of the photosensitive drum.